From what I understand, when we apply traditional pigments (i.e., non neon) on paper, we make the paper reflect less light than it was before we applied it.
That is, when we apply a pigment on paper, we are actually putting a filter on paper that absorbs the complement of the color we think we are coloring with, which in almost all cases reduces the amount of light reflected.
This is most obvious when we apply black pigment on paper, since it absorbs lots of light, the paper gets darker and darker as we apply more and more pigment.
What confuses me is the opposite seems to be true for what we call "bright colors", like red, yellow, sky blue, magenta, etc. That is, we think we are making the paper brighter by applying more pigment, which surely can't be the case, since that's like saying we're making a black paper darker while drawing through a camera on negative, where the white is inverted to black.
This is specifically annoying for me for yellow because yellow seems really really bright when it is saturated on paper, whereas paper should be much brighter than yellow because it is white, and pale hues of yellow should be brighter than yellow because they have less blue prevented from hitting your eyes from them. It is really tempting to say that we want a thick layer of yellow to make something brighter, but how does this make any sense?
I mean I have some guesses that if the pigment is applied flat enough, it might behave kind of like a mirror or glossy paper, and reflect light in a more concentrated way rather than scattering it, and make more light hit your eyes than a regular painting surface, which tends to have a tooth to it, but I am not sure how much this contributes to the question.
Can somebody explain why we think we are making the paper brighter by applying what we perceive to be bright colors, but darker when we apply what we perceive to be dark colors, when surely in both cases we are making the paper darker? Or what's more confusing, it seems like we're making it lighter until a certain point, after which it seems like we're getting darker, especially for really strong pigments that look black at full saturation like Phthalo Blue, Prussian Blue, and Phthalo Green.